BDNF and GDNF expression in discrete populations of nociceptors
Introduction
Primary sensory neurons of the dorsal root ganglia (DRG) are a heterogeneous population of neurons which detect and transduce information from a variety of specialized receptors, such as nociceptors, mechanoreceptors, and proprioceptors (Basbaum et al., 2009). Differences among DRG sensory neurons are the cell body size, physiological properties, neurochemical phenotypes, and central/peripheral projections (Lawson, 1992, Snider and McMahon, 1998).
Nociceptors are small-to-medium size neurons and, based on their neurochemical content, are classically divided into peptidergic and non-peptidergic nociceptors, respectively, which express neuropeptides such as the calcitonin gene-related peptide (CGRP) and substance P (SP; McCarthy and Lawson, 1990), or typically bind the isolectin B4 (IB4; Silverman and Kruger, 1990).
Nociceptors depend on neurotrophic factors for their survival and differentiation (Klein, 1994; Buj-Bello et al., 1995; Snider and Silos-Santiago, 1996, Golden et al., 2010). However, increasing evidence indicates that neurotrophic factors are also able to directly and timely challenge neuronal excitability. In particular, the brain-derived neurotrophic factor (BDNF) and the glial-derived neurotrophic factor (GDNF) are widely implicated in the control of nociceptive neurotransmission under normal and pathological conditions (Bennett, 2001, Boucher and McMahon, 2001, Merighi et al., 2008a, Merighi et al., 2008b, Merighi, 2015). Both are synthesized in small-to-medium size DRG neurons, coexist with several neuropeptides, and are anterogradely transported to the spinal cord dorsal horn (Zhou and Rush, 1996, Michael et al., 1997, Holstege et al., 1998, Ohta et al., 2001, Rind and von Bartheld, 2002, Salio et al., 2007, Salio et al., 2014).
The precise functional role of BDNF and GDNF on the transmission of nociceptive information still remains controversial as both antinociceptive and pronociceptive effects have been reported depending on the type of stimulus and the pathological pain model adopted. However, numerous studies in the last decade have importantly contributed to uncovering specific mechanisms which seem to follow a general scheme: while BDNF enhances the release of excitatory neurotransmitters (glutamate, SP, and CGRP) and contributes to inflammatory and neuropathic pain (Coull et al., 2005, Lin et al., 2011, Bao et al., 2014, Chen et al., 2014, Zhang et al., 2014), GDNF reduces the central release of glutamate from activated nociceptors (Salio et al., 2014) and exhibits antinociceptive effects in different neuropathic pain models (Boucher et al., 2000, Sakai et al., 2008, Kimura et al., 2015).
A better knowledge of the cellular localization of BDNF and GDNF in nociceptive pathways is an important requirement for a better interpretation of their specific function. In the present study, we analyzed the expression of BDNF and GDNF in DRG nociceptors and investigated the pattern of their co-localization with established markers of nociceptor subpopulations.
Our results demonstrate the existence of two distinct subpopulations of peptidergic CGRP-positive nociceptors, one co-expressing BDNF and SP, the other GDNF and somatostatin (SST). The discrete expression of these neurotrophic factors into defined subpopulations of nociceptors suggest that they may be differently implicated in the discrimination of specific painful stimuli modalities.
Section snippets
Animals
Immunohistochemical studies were carried out on male 3-week old CD1 mice (Light microscopy (LM): n = 3; Electron microscopy (EM): n = 3 for Araldite embedding; n = 3 for Lowicryl embedding). Experiments were performed according to EU and national regulations on animal welfare and have been authorized by the Italian Ministry of Health (600.8/82.20/AG1826).
Tissue preparation for light and electron microscopy
Under deep pentobarbital anesthesia (60 mg/100 g), mice were perfused with Ringer solution followed by cold fixative solution. The latter consisted of
Expression of BDNF and GDNF in nociceptors
BDNF- and GDNF-immunoreactivity (IR) was distributed in small-to medium-sized DRG neurons (Fig. 1A and B). No double-labeled neurons were observed (Fig. 1C and Supplementary Fig. 1), indicating that BDNF and GDNF were expressed in distinct populations of DRGs. The number of IR-cells per DRG was counted and BDNF resulted three time more expressed than GDNF (186.4 ± 1.7 BDNF-IR cells/DRG vs 57.7 ± 0.3 GDNF-IR cells/DRG, n = 3 mice). The variances of the numbers of both BDNF and GDNF neurons across
Discussion
The classification of DRG neurons and, in particular, of nociceptors has undergone considerable refinement during the last decades (Le Pichon and Chesler, 2014).The seminal distinction between peptidergic and non-peptidergic neurons has been progressively split into a number of ramifications leading to the identification of several phenotypically distinct neuronal subpopulations.
In the present study, we have updated nociceptor classification in terms of their “neurotrophic content”. In
Conclusions
Detecting the phenotypic diversity of nociceptors represents a necessary prerequisite for comprehensive understanding of their function in pain transmission. Here we have demonstrated that the neurotrophic factors BDNF and GDNF identify two discrete subpopulations of peptidergic nociceptors which project independently to the spinal dorsal horn. The presence of different subsets of peptidergic nociceptors suggests the existence of a sophisticated system to encode specific nociceptive stimuli and
Acknowledgments
This work was supported by a grant from Compagnia San Paolo(Fondi di Ateneo 2012; identification: Salcaten12).
References (46)
- et al.
Cellular and molecular mechanisms of pain
Cell
(2009) - et al.
The somatostatin analogue octreotide inhibits capsaicin-mediated activation of nociceptive primary afferent fibres in spinal cord lamina II (substantia gelatinosa)
Eur. J. Pain
(2011) - et al.
Neurotrophic factors and neuropathic pain
Curr. Opin. Pharmacol.
(2001) - et al.
GDNF is an age-specific survival factor for sensory and autonomic neurons
Neuron
(1995) - et al.
Vanilloid receptor-1(TRPV1)-dependent activation of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse
Pain
(2007) - et al.
Ultrastructural localization of brain-derived neurotrophic factor in rat primary sensory neurons
Neurosci. Res.
(2001) - et al.
Cell type and conduction velocity of rat primary sensory neurons with calcitonin gene-related peptide-like immunoreactivity
Neuroscience
(1990) - et al.
Ultrastructural visualization of glutamate and aspartate immunoreactivities in the rat dorsal horn, with special reference to the co-localization of glutamate, substance P and calcitonin-gene related peptide
Neuroscience
(1991) - et al.
BDNF as a pain modulator
Prog. Neurobiol.
(2008) Substantia gelatinosa of the spinal cord
Involvement of neural cell adhesion molecule signaling in glial cell line-derived neurotrophic factor-induced analgesia in a rat model of neuropathic pain
Pain
Tackling pain at the source: new ideas about nociceptors
Neuron
NK1 receptor activation leads to enhancement of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse
Pain
PAR2-mediated upregulation of BDNF contributes to central sensitization in bone cancer pain
Mol. Pain
BDNF-mediated modulation of GABA and glycine release in dorsal horn lamina II from postnatal rats
Dev. Neurobiol.
Neurotrophic factors: important regulators of nociceptive function
Neuroscientist
Science
Glutamate receptor delta2 subunit in activity-dependent heterologous synaptic competition
J. Neurosci.
Presynaptic GABAergic inhibition regulated by BDNF contributes to neuropathic pain induction
Nat. Commun.
BDNF from microglia causes the shift in neuronal anion gradient underlying neuropathic pain
Nature
RET signaling is required for survival and normal function of non peptidergic nociceptors
J. Neurosci.
Immunocytochemical localization of GDNF in primary afferents of the lumbar dorsal horn
Neuroreport
The molecular dynamics of pain control
Nat. Rev. Neurosci.
Cited by (15)
The emerging role of the BDNF-TrkB signaling pathway in the modulation of pain perception
2020, Journal of NeuroimmunologyBalneotherapy decreases mechanical hyperalgesia by reversing BDNF and NOS2 immunocontent in spinal cord of mice with neuropathic pain
2020, Journal of NeuroimmunologyCitation Excerpt :Thus, we confirm the effectiveness of balneotherapy in neuropathic pain by decreasing BDNF-related nociception as well as its immunocontent in the spinal cord. Studies have reported an increase in BDNF expression after nerve injury in a population of small to medium-diameter DRG neurons (Salio and Ferrini, 2016). In addition, studies have demonstrated the peripheral pro-nociceptive role of BDNF.
The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord
2018, Progress in NeurobiologyCitation Excerpt :In mouse, GDNF-IR fibers in SGR originated from a subpoplation of small-to medium-sized DRG neurons totally separated and numerically much restricted (about one third) than that expressing BDNF (Salio and Ferrini, 2016). In turn, virtually all GDNF-IR DRG neurons also expressed CGRP, and a substantial fraction of them (about 70%) was SST-IR (Salio and Ferrini, 2016). Immunoreactivity in spinal cord was detected in lamina IIi.
Distribution and chemical coding of sensory neurons innervating the skin of the porcine hindlimb
2017, NeuropeptidesCitation Excerpt :In the present study we have found that the vast majority of SOM-IR neurons in the porcine DRGs were small- or medium-sized, while only a few of the large-sized neurons were expressing this peptide (exclusively in lumbar DRGs). Since a fraction of SOM-containing DRG neurons has been shown to co-express SP (cat; Hanesch et al., 1995), CGRP (mouse; Salio and Ferrini, 2016) and/or GAL (cat; Garry et al., 1989), it appears that SOM may also be involved in nociceptive transmission. However, available data concerning the putative involvement of SOM in nociception appear to be somewhat contradictory, as this peptide has simultaneously been implicated as possessing both the pro- (Kuraishi et al., 1985; Morton et al., 1989) and anti-nociceptive properties (Helyes et al., 2000; Taddese et al., 1995).